Generally, in optical information recording media such as optical discs, information is linearly recorded on tracks as a reference. In the optical discs, tracks are spirally or concentrically formed. The interval between the tracks adjacent to each other is called a track pitch. The tracks are physically formed in the form of a groove, for instance. Information is recorded or reproduced by causing a light spot to track the groove. Further, as another physical shape of tracks, pits called sample servo pits may be linearly and periodically formed. In the latter case, information is recorded or reproduced by causing a light spot to trace a virtual track based on reflected light from the pits.
In an optical disc, a large number of tracks are formed adjacent to each other in a radial direction of the disc. Recording marks representing recorded information are formed independently of each other on each of the tracks. A recording mark recorded on a certain track may erase or overwrite a recording mark on a track adjacent to the certain track. These phenomena are also called “cross erase”, and are not preferable because these phenomena make it impossible to accurately reproduce the information on the adjacent track. In view of the above, the track pitch is set to a sufficiently large value to avoid the influence of cross erase.
On the other hand, in the field of magnetic recording, there is proposed a recording system called “Shingle write” for the purpose of reducing the track pitch (see e.g. patent literature 1). In the above system, for instance, information is recorded on tracks adjacent to each other successively from the inner peripheral side of a disc to the outer peripheral side of the disc. Further, a part of a recording area in a certain track is overwritten by a recording area in a track adjacent to the certain track. By the above technique, it is possible to set a track pitch smaller than the width of a recording area on which information is actually recorded by a magnetic head.
However, in the case where the shingled write recording as described above is applied to optical recording, information recorded in an optical information recording medium may not be accurately reproduced. This drawback is described in the following.
A first issue to be considered is the size of an area to be actually rewritten in performing a rewriting operation. Specifically, in the case where a rewriting operation is performed in a certain track of a rewritable optical information recording medium, the area (hereinafter, referred to as a “recording area”) to be actually rewritten is made larger than the area, on which recording marks and spaces are formed. For instance, in recording on an area containing a phase-change material, there is a region, in the periphery of recording marks, from which the originally recorded information may be erased. This is because a crystal region which has undergone recrystallization after melting is formed in the periphery of recording marks. The crystal region may erase the information recorded in the adjacent track.
In addition to the above, there is another issue to be considered, namely, the width of a recording area is not constant. Specifically, in optical recording, the lengths of a recording mark and a space to be formed are determined by changing the irradiation power of laser light. For instance, in the case where the material of a recording film of an optical information recording medium is a phase change material, the laser power is set to a highest power level (this is called a write power) at the time of forming a recording mark, and the laser power is set to a relatively low power level (this is called an erase power or a space power) at the time of forming a space. The length of the recording mark is adjusted by irradiating pulse light of a write power for a predetermined time duration by a predetermined number of times.
This means that the amount of heat to be applied from laser light to a recording film varies depending on the pattern of information to be recorded. Specifically, the amount of heat to be applied at the time of forming a space is smallest. Further, the amount of heat to be applied at the time of forming a recording mark having a short code length is larger than the amount of heat to be applied at the time of forming a space, and the amount of heat to be applied at the time of forming a recording mark having a long code length is further larger than the amount of heat to be applied at the time of forming a space. As the amount of heat to be applied to a recording film on a certain track increases, the amount of heat to be transferred to the adjacent track increases. This results in an increase of the width of a recording area, and results in an increase of the width of the adjacent track in which data is rewritten.
As described above, if the shingled write recording is applied to optical information recording, the shape of a recording mark formed in a certain track may vary depending on the pattern of information recorded in a track adjacent to the certain track. This makes it difficult to accurately reproduce the information recorded in the optical information recording medium.